Stainless steel bars and rods of improved cross-sectional hardness uniformity



United States Patent 3,476,616 STAINLESS STEEL BARS AND RODS OF IMPROVED CROSS-SECTIONAL HARDNESS UNIFORMITY Edward J. Dulis, Mount Lebanon Township, Allegheny County, Pa., assignor, by mesne assignments, to Crucible Inc., a corporation of Delaware No Drawing. Filed Sept. 1, 1966, Ser. No. 576,529 Int. Cl. C21d 7/14 US. Cl. 148- 12 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates to bars or rods of metal having a face-centered-cubic crystal structure, such as austenitic stainless steel, and in particular, to bars or rods of such metal for use in automatic screw-making machinery or in other applications wherein the machinability of the metal is important. Still more particularly, the invention concerns bars or rods of the kind above-indicated that exhibit a relatively uniform hardness across the crosssectional area thereof, and to a novel method of making such bars or rods.

BACKGROUND OF THE INVENTION In the as-annealed condition, bars or rods of austenitic stainless steel exhibit a relatively unifrom hardness profile, ranging, for example, from 190 BHN at the edge to 170 BHN at the center. In that condition, however, long-time experience in commercial machining operations has shown that the steel is too soft to machine well in many instances because it is somewhat gummy. Hence, it has hitherto been the practice to cold-draw the steel at room temperature to the extent of about 6% reduction in cross-sectional area to increase the hardness to about 220 BHN at the midradius. This practice works the steel so that it imparts a relatively excessively hard surface (about 290 BHN). The cold-drawing operatiton raises the hardness of the center to about 200 BHN. Points between the center and the edge have hardness between those values. Now the hardness of the edge is undesirably high. The high surface hardness is related to lower lathe machinability rating and poorer chip rating as will be shown hereinbelow (see Table IV). The bar or rod is ground to remove a surface layer, for example, about 0.003 inch thick. After being ground, the bar or rod is still undesirably nonuniform in hardness across its crosssectional area. Such, however, is the nature of the bars or rods of austenitic stainless steel now commonly on the market.

DESCRIPTION OF THE INVENTION In accordance with the invention, it has been discovered that if the bars or rods are heated to a temperature of about 200 to 500 F. before being subjected to a final drawing operation, much more favorable results are obtained. When this is done, it is possible to obtain. rods that exhibit, for example, a hardness of 213 BHN at the center and about 250 BHN at the edge. This invention is of considerable importance to a manufacturer of such bars 3,476,616 Patented Nov. 4, 1969 and rods; if it is desired to produce a rod of a given predetermined diameter with a specified surface hardness limit, it is much easier to do so with the bars produced by practicing the method of the instant invention than with those of the prior art. The bars or rods of the prior art exhibit a sharp hardness gradient in the vicinity of their edges, and it sometimes happens that when the bar has been machined or ground to the desired. dimensions, its surface hardness is unsatisfactory. As indicated above, this undesirable hardness gradient will :be far less likely to occur with bars or rods made in accordance with the instant invention.

The present invention encompasses a method wherein face-centered-cubic crystal structured stainless steels are plastically deformed in a temperature range of about 200 F. to the recrystallization temperature, the latter temperature varying in accordance with the amount of deformation and the temperature. Basically, the mechanism involves the warm working of the face-centeredcubic austenite to impart a depth hardening by austenite deformation or by austenite deformation. plus minimized deformation-transformation product. Thus, both the stably austenitic stainless steels, which remain completely austenitic after severe deformation at room temperature, and the unstably austenitic stainless steels, which undergo partial austenite deformatiton hardening and partial austenite-to-martensite transformation as a result of room-temperature transformation, are amenable to the development of improved hardness uniformity across the cross-section of bar sizes by the practice of the present invention. Significantly improved hardness uniformity can be achieved by warm plastic deformation of the various types of commercially used austenitic stainless steels. Studies were made on types 301, 304, 310, 316, 321 and 303 austenitic stainless steels. The results, presented hereinbelow in Table I, clearly show that a substantial improvement in cross-sectional hardness gradient is achieved in steels that are hardened by the plastic deformation of austenite only, as is shown by the lack of any magnetic response, a test that is commonly used to indicate the occurrence of transformation from anstenite to deformation-induced martensite in the type 310 steel (I. B. Austin and D. S. Miller, Magnetic Permeability of Some Austenitic Iron-Chromium-Nickel Alloys as Influenced by Heat Treatment and Cold Work, Trans. of the ASM, September 1940, pages 743-755, showed that type 310 does not form deformation-induced martensite even after severe deformation at room temperature). Also, type 301 steel, which is very likely to undergo transformation from austenite to deformationinduced martensite, has, in addition to the hardening mechanism of austenite deformation, a deformation-transformation-martensite formation, the latter reaction being partially suppressed by the warm deformation of the instant invention. Thus, regardless of whether the greater depth of uniform hardening attributable to warm drawing is related to austenite deformation only or to austenite deformation plus partial austenite to deformation-martensite formation, the degree of decrease in cross-sectional hardness gradient is clearly demonstrated.

Bar Composition (percent) Diameter Alloy (in.) Mn P S Si Ni Cross- Actual Hardness (BHN) at Distance Below the Surface Sectional Reduction Drawing Hardness of area Temp. Magnetic 0. 500 0. 375 0.282 0.250 0. 188 0. 130 0. 004 0. 030 Gradient Alloy (percent) F.) Responc in. in. in. in. in. in. in. in. (BHN) 8.4 RT Very strong. 233 307 74 19. 1 Strong 217 545 28 3. 59 39 T304 15. a 350 ht 22s 11 it 28 1 5 17 T310 25. 2 RT 275 28 26.2 do 250 13 15. 250 28 T316 1%. gar 233 13 1 268 33 T321 a Diamond pyramid hardness values converted to BHN. Hardness results the average of five tests.

The invention finds use in the manufacture of bars, rods, etc., ranging in size from A1 in. to as high as 4 in., although most typically it is used in making rounds, hexes, squares and other shapes in the vicinity of /2 to 2 in.

The invention finds use in making bars or rods of any metal having an austenitic (face-centered cubic) crystal structure, including certain cobalt-base or nickel-base super-alloys, where hardness increase by cold work plus hardness uniformity are important (for example, for material for fastener applications), but it is most coinmonly useful with reference to the austenitic stainless steels of the 200 series and the 300 series. The new process is particularly useful with the free-machining grades of austenitic stainless steel, such as AISI type 303 and AISI type 303-SE stainless steel.

The temperature to which the steel is warmed before being cold drawn may, as indicated above, range from about 200 F. to 500 F., or possibly somewhat higher temperatures up to about 800 F. may be used, but in any event I use a temperature below the recrystallization and the carbide-precipitation temperatures of the metal. Problems related to carbide precipitation, such as reducing corrosion resistance (particularly intergranular corrosion resistance), occur during heating in the range about 900 to l300 F., so that this temperature range is avoided. Apart from making it somewhat easier to effect heavier draws, the use of higher temperatures within that range has little to recommend it in comparison with the use of lower temperatures in the range. In addition, at higher temperatures, there may be difficulty in finding a lubricant of adequate stability.

The extent of reduction in area of the bar or rod as a result of the drawing operation may range from about 3% to about 20% Smaller reductions tend to increase the center and mid-radius hardness considerably less than larger reductions. Henceffor most purposes, it is preferred to use a draw that reduces the cross-sectional area by about 5 to 8%.

As a specific example of the practice of the invention, samples from a commercial heat of free-machining austenitic stainless steel of the following composition were investigated: 0.09% carbon, 3.20% manganese. 0.019%

phosphorus, 0.33% sulfur, 0.51% silicon, 7.82% nickel, 17.99% chromium, 0.05% vanadium, 0.20% molybdenum, 0.17% copper, balance iron. The steel was cast into a commercial-sized ingot, hot-rolled to a cross section of about 1 in. round, and annealed. Upon being heated to 300 F. and then being warm-drawn to the extent of 5.17% reduction in area, the steel exhibited a center hardness of 205 BHN and an edge hardness of 247 BHN. For purposes of comparison, the same steel, drawn to the extent of 4.25% reduction in area at room temperature, exhibited a center hardness of 196 BHN and edge hardness of 275.5 BHN.

Further data relating to tests with the same steel and demonstrating the effect of the invention are presented in the following Table II.

TABLE II BHN Reduc- Draw tion in 0. 030 Temper- Area, Midfrom ature Percent Center Radius Radius Radius Edge R T 4. 25 196 196. 5 200 220 275. 5 10. 237 242 257 288 293 4. 53 197 197. 8 203 222 256 200 F 7. 52 213 216. 4 222 285. 5 250 11. 52 231 231. 5 237. 5 251. 6 260 5. 17 205 205. 5 205. 8 220. 5 247 0 F 7. 88 215 215. 5 217 240 250 11. 67 226 227 230. 5 244. 5 261 15. 77 240 241. 8 248 257 264 5. 35 209 200 214 230 258 0 F 7. 88 215 223. 5 238 240 248 11. 97 231. 5 232 237 248. 7 261 16. 01 243. 5 243. 5 245. 5 256 261. 5 5. 53 200 200. 5 204. 8 220 239 0 F 8. 42 215 216 220. 5 234. 5 247 12. 18 232 233. 2 238. 3 252. 5 258. 5 16. 13 243. 5 243. 6 248 256. 7 261. 5

Moreover, with respect to another commercial 15-ton heat of free-machining austenitic stainless steel consisting of 0.09% carbon, 1.68% manganese, 0.254% sulfur, 0.032% phosphorus, 0.40% silicon, 8.49% nickel, 17.12% chromium, 0.031% nitrogen, balance iron except for impurities, data were obtained that indicate the superiority in cross-sectional hardness uniformity and machinability of rods of austenitic stainless steel treated in accordance with the invention in comparison with cold-drawn rods of the same steel.

TABLE III Machinebility Rating Brinell Hardness Number Draw Draw Distance below Surface (Inch) Lathe Sample Percent Temp., Speed, Mach. 0111p Number R.A. F. Lpm. 0. 030 0. 125 0. 250 0. 375 0. 500 Rating 8 Rating ANN 195 183 171 175 168 101 1 ANN 178 177 174 170 171 101 1 6 2 RT 26 281 263 236 226 218 94 4-5 6 9 RT 50 293 259 238 226 221 79 6 7 RT 286 262 235 226 219 6 2 RT 50 278 259 232 222 219 82 4 6 3 RT 50 286 260 238 227 223 6 l RT 286 262 235 226 219 89 5 6 200 26 244 4 221 215 214 92 3-4 6 200 50 249 237 222 216 216 98 4 6 200 70 242 233 221 215 213 99 4 6 360 26 243 236 227 221 220 96 4 6 350 50 240 229 222 218 217 92 3-4 6 350 50 244 226 212 209 209 93 4 6 350 70 245 239 226 218 216 6 350 70 246 235 222 216 215 97 4 6 350 70 2 227 217 213 212 98 3 10 3 RT 50 285 267 252 246 83 6 10 350 50 247 241 236 238 4-5 1 Reduction of Area.

2 Room Temperature.

' Higher rating indicates better machinability. 4 Higher chip rating indicates poorer chip.

Data with respect to further tests conducted with the nickel, and 17.99% chromium, balance iron except for steel consisting of 0.09% carbon, 3.70% manganese, impurities, are presented in the following Tables IV 0.33% sulfur, 0.019% phosphorus, 0.55% silicon, 7.82% 40 and V.

TABLE IV.--CROSS-SECTIONAL HARDNESS OF GOLD AND WARM DRAWN TYPE 303 PLUS X Cross- Actual Drawing Brinell Hardness Number I Sectional Percent Temper- Distance below the Surface Hardness Reduction ature, Gradient, Area F. 0.030 0. 0.250 0.375 0. 500 BHN 1 RT 286 221 204 198 198 88 200 256 223 208 204 202 54 300 262 228 213 201 201 61 400 257 230 214 209 209 48 500 258 221 206 202 200 58 1 RT 262 247 220 217 214 48 200 247 238 224 218 218 29 300 246 238 218 216 213 33 400 238 233 321 217 217 21 500 244 235 220 213 211 33 1 RT 290 289 261 250 242 48 200 260 258 247 238 234 26 300 258 255 243 238 237 21 400 253 250 244 236 236 500 260 252 246 230 235 25 300 263 257 248 242 240 23 400 262 256 245 243 243 19 500 262 258 250 243 243 19 190 183 184 186 185 7 286 255 236 225 222 64 e Hardness results are the average oi five tests. I Room Temperature.

3 Mill Anneal.

I Mill Drawn.

TABLE V.-DRILL AND LATHE MACHINABILITY OF WARM AND COLD DRAWN TYPE 303 PLUS X Drill Lathe Machinability Actual Percent Drawing Machin- Reduction Temperaability Chip of Area ture, F. Rating Rating A Rating b Room 103 94 6 Room 103 89 4 Room 101 81 7 a Higher rating indicates better machinability. b Higher rating indicates a poorer chip.

0 Mill Anneal.

d Mill Drawn.

While I have shown and described herein certain embodiments of the invention, I intend to cover as well any change or modification therein which may be made without departing from its spirit and scope.

a reduction in cross-sectional area of from 3% to 20% v at a temperature between 200 F. and the recrystallization temperature of said steel.

2. A method as defined in claim 1, characterized in that said metal comprises a free-machining grade of austenitic stainless steel and is warm-worked to the extent of a reduction in cross-sectional area of between 5 and 8% at a temperature of 200 to 500 F.

References Cited UNITED STATES PATENTS 2,716,080 8/1955 Schwarz 148-12 2,767,837 10/ 1956 Nachtman et a1. 148-12 2,924,543 2/1960 Nachtman 148-12 2,924,544 2/1960 Nachtman 148-12 3,001,897 9/1961 Nachtman 148-12 L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,476 ,616 November 4 1969 Edward J. Dulis It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Columns 3 and 4, TABLE I, seventh column, line 3 thereof, "0.56" should read 0.66

Signed and sealed this 21st day of April 1970.

(SEAL) Attest:

Edward M. Fletcher, J r.

Commissioner of Patents Attesting Officer WILLIAM E. SCHUYLER, JR. 

